Flexographic printing plates are well known for use in relief printing on a variety of substrates such as paper, corrugated board, films, foils and laminates. Flexographic printing plates can be prepared from photosensitive elements comprising a photopolymerizable layer containing an elastomeric binder, a monomer, and a photoinitiator, interposed between a support and a cover sheet or multilayer cover element. A preferred process of making such photosensitive elements is described in U.S. Pat. No. 4,460,675 where a previously extruded photopolymerizable composition is fed into the nip of a calender and is calendered between a support and a multilayer cover element to form a photopolymerizable layer. Upon imagewise exposure of the photosensitive element with actinic radiation through a photomask, the exposed areas of the photopolymerizable layer are insolubilized. A common technique for bringing a photosensitive element and a photomask into close contact with one another is to juxtapose the element and mask, and draw a vacuum from between them usually by use of a vacuum frame. Treatment with a suitable solvent or thermal treatment combined with contacting an outermost surface of the element to an absorbent surface after imagewise exposure removes the unexposed areas of the photopolymerizable layer leaving a printing relief which can be used for flexographic printing. Such materials are described in U.S. Pat. No. 4,323,637; U.S. Pat. No. 4,427,759; and U.S. Pat. No. 4,894,315. Thermal development of photosensitive elements to form flexographic printing plates is described in U.S. Pat. No. 5,015,556; U.S. Pat. No. 5,175,072; U.S. Pat. No. 5,215,859; and WO 98/13730.
Digital methods and associated recording materials that do not require a separate photomask have been developed and are described in WO 94/03838, WO 94/03839, WO 96/16356, and EP 0767 407. Such recording materials comprise a conventional photopolymerizable layer, as previously described, and additionally a layer capable of forming an integrated photomask. The additional layer is sensitive to infrared radiation and opaque to actinic radiation, a so-called infrared sensitive or IR-sensitive layer. This infrared sensitive layer is imaged digitally, whereby the infrared sensitive material is imagewise vaporized or transferred to a superposed film. Subsequent overall exposure of the photosensitive element through the resulting integrated photomask, washing off unpolymerized areas and remaining areas of the infrared sensitive layer, and drying the element yield a flexographic printing plate. Another method to produce flexographic printing plates is by imagewise ablating with a laser parts of a laser-engravable, reinforced elastomeric material thereby forming a printing relief. Such materials and processes are described in U.S. Pat. No. 5,798,202, U.S. Pat. No. 5,804,353 and EP 1 215 044. These digital methods are used for the preparation of flexographic printing plates in sheet form or in cylindrical form.
In the flexographic printing process, the flexographic printing plate is mounted on a printing cylinder (plate cylinder) and the raised parts of the three-dimensional relief formed in the surface of the flexographic printing plate are pressed against an inking unit (called Anilox) in order to be inked on their top surface. Thereafter, the inked raised areas are pressed against a substrate such as paper, foil, etc mounted on an impression cylinder. As the flexographic printing plate and Anilox or substrate are adjusted and limited mechanically, it is the height of the raised parts of the flexographic printing plate's surface that determines the amount of physical impression between flexographic printing plate and Anilox or flexographic printing plate and substrate. Relief areas that are raised higher than others will produce more impression than those that are lower or even recessed. Flexographic printing plates can show small differences in relief heights (up to 25–30 μm) due to tolerances of the thickness of the raw photosensitive elements and/or caused by their production process. Therefore, the flexographic printing process is usually quite impression-sensitive, i.e., more or less intense contact between flexographic printing plate and Anilox/substrate may impact the print result quite drastically, which is why impression has to be controlled carefully. If the impression is too high, some image areas can be squeezed. Otherwise, if the impression is too low, the ink transfer is insufficient. In both cases, the quality of the resulting flexographic printing images is bad.
An objective judgment of the most suitable impression for all relief areas of a flexographic printing plate is difficult to obtain because of these varying relief heights. So, the impression settings have to be tested individually for every flexographic printing plate in use. It is a question of the pressman's talent and the utilized press precision as to how standardized and reproducible are the chosen settings.
Therefore, it is an object of the present invention to avoid this trial and error procedure for controlling the impression in flexographic printing processes and to provide an easy method for controlling the quality of the resulting prints.